Trucks and other ground vehicles have a hydraulic power steering system that provides power assist in turning the steerable wheels of the vehicle. The power steering system drives the steerable wheels move along a steering stroke. The ends of the steering stroke are defined by axle stops that mechanically prevent further movement of the wheels.
FIG. 23 illustrates in simplified form the front steerable wheels WL, WR of a ground vehicle, the wheels connected by a steering linkage S. When the wheels are steered to the right as viewed in FIG. 23, the end of the steering stroke is defined by an axle stop AL. When the wheels are steered to the left, the end of the steering stroke is defined by an axle stop AR. In the illustrated embodiment the axle stops AL, AR are formed from threaded bolts that extend from the vehicle frame. Other axle stop designs are known in the ground vehicle steering art.
A conventional hydraulic power steering system flows high-pressure power steering fluid to a fluid motor that has a piston within a closed hydraulic cylinder. The piston divides the cylinder into motor chambers on opposite sides of the piston. The piston is connected to a steering linkage that moves the steerable wheels along the steering stroke. The piston is axially movable in the cylinder between opposite ends of a piston stroke to actuate the steering linkage and move the steerable wheels along their steering stroke to the left or right.
To initiate a turn, the driver turns a steering wheel to cause the steerable wheels to move in the desired turning direction. The steering wheel is connected to a control valve that flows high-pressure fluid into one of the motor chambers (the “high-pressure chamber”) and connects the other motor chamber (the “low-pressure chamber”) to an exhaust. The fluid pressure in the high-pressure chamber generates power assist moving the piston from its centered position in the cylinder (corresponding to the centered, straight-ahead position of the steerable wheels along their steering stroke) towards the low-pressure motor chamber. This actuates the steering linkage, moving the steerable wheels in the turning direction.
It is desirable to remove power assist before the steerable wheels hit the axle stops. If the power steering system powers the steerable wheels against an axle stop, severe mechanical strain is placed on the steering linkage, the power steering components, and other component parts of the vehicle.
Conventional hydraulic power steering systems remove power assist before the steerable wheels reach the axle stops. The piston stops moving at an end of its piston stroke when the power assist is relieved. The end of the piston stroke occurs before the steerable wheels impact the axle stops.
To relieve power assist when the piston reaches an end of its piston stroke, a normally closed fluid line connects the two motor chambers. In many conventional systems the fluid line extends through the piston. Normally-closed check valves are located on opposite ends of the fluid line adjacent the motor chambers.
The fluid pressure in the high-pressure motor chamber opens the check valve adjacent the high-pressure motor chamber, but the fluid pressure transmitted through the fluid line from the high-pressure motor chamber urges the other check valve closed. As a result, the fluid line remains closed between the two motor chambers despite one check valve being open as the piston moves along its piston stroke from the centered position towards one end of the cylinder.
When the piston nears the end of its piston stroke, signaling that the steerable wheels are nearing the axle stops, an actuating member in the low-pressure chamber opens the check valve adjacent the low-pressure chamber. The fluid line now opens on both sides of the piston, equalizing the pressure in the two motor chambers and stopping the piston at the end of its piston stroke before the steerable wheels hit the axle stops.
If the steering wheel is now turned to return the steerable wheels towards their centered or straight-ahead position, the control valve connects the one motor chamber to exhaust and connects the other motor chamber to the high-pressure fluid. The check valve adjacent the one motor chamber closes from the loss of fluid pressure in the chamber, closing the fluid line and enabling the high pressure in the other motor chamber to reverse the motion of the piston. The piston begins moving towards the other end of the hydraulic cylinder and moves the steerable wheels away from the axle stops towards their centered position.
The method of relieving power assist by opening a fluid line between the two motor chambers for direct fluid communication between the two chambers has worked well for many years. Power steering systems have conventionally used an engine-driven pump to continuously flow high-pressure fluid to an open-center control valve. The open-center control valve continuously flows the power steering fluid received from the pump, even when the wheels are in a straight-ahead position and are not being turned. The continuous flow of high-pressure fluid through the control valve enables power assist to be quickly re-established after being relieved due to the piston reaching an end of its piston stroke. The power steering pump limits the flow rate through the system so that fluid flow through the system when the fluid line is open or when the check valves close does not damage system components.
Today, however, an increasing number of trucks use energy-saving power steering systems that utilize a closed-center control valve to control flow to the steering motor. A closed-center valve shuts off the flow of high-pressure fluid into the control valve when the valve is in a centered condition with the wheels not being turned. The control valve allows high-pressure fluid to flow through the valve and to the fluid motor only when the control valve is away from its centered position for turning.
Because a steering system utilizing a closed-center valve does not require a continuous flow of high-pressure fluid, power steering fluid is provided to the control valve when needed from a gas-pressurized accumulator. An electric motor intermittently supplies power steering fluid to the accumulator from a reservoir on an as-needed basis when the fluid volume or fluid pressure in the accumulator drops below some minimum level.
Opening a fluid line between the motor chambers to equalize fluid pressures and relieve power steering will work with a closed-center type power steering system, but does not work well. The fluid flow through the motor chambers rapidly depletes the energy stored in the accumulator, depleting the accumulator within a few seconds. The result is a time delay until the accumulator is recharged and power steering assist can be reestablished. During the delay, steering would be manual and unsatisfactory.
Furthermore, when the fluid line is open between motor chambers the flow of fluid discharged from the accumulator is not regulated by the power steering system. A high flow rate through the piston may damage one or both of the check valves, preventing the valve from closing.
Thus there is a need for an improved method of relieving power assist from a power steering system before the steerable wheels of a vehicle reach the axle stops. The method should limit the rate energy is depleted in the system so that power assist can be quickly reestablished, and regulate the flow of fluid through the system while power assist is being relieved to avoid damage to system components.